The Independent Police Complaints Commission is once more under the spotlight. Its investigators are operating under a heavy workload, examining a series of controversial deaths after police operations, none more so than the fatal shooting of Mark Duggan.

MPs and commentators are calling on the IPCC to be robust, efficient and blazingly independent, but when its deputy chair, Deborah Glass, gave evidence to a Commons select committee this week, she seemed to procrastinate and flounder in the face of simple questions that required direct answers, leading to a reprimand from the committee chairman, Keith Vaz.

Not for the first time since its former head departed 18 months ago, the IPCC looked like an organisation in desperate need of a new leader but it is perhaps now, more than ever, that the absence of a fresh approach at the top is most obvious.

The home secretary, Theresa May, appoints the head of the IPCC, but despite repeated queries to the Home Office, no one is able to explain the long delay in filling the post. The interim chair, Len Jackson, who has been with the IPCC since its inception, told MPs he would stay on until the end of October, in the hope that someone would be appointed by then.

Jackson has attempted to explain the difficult nature of the work he and his colleagues do, as the IPCC carries out a record number of independent investigations, and deals with public criticism from David Lammy, the MP for Tottenham, and the family of Mark Duggan.

"Relationships with families and with the community are always difficult, particularly in the early stages of an investigation such as this one," he said.

"We have critics on both sides of the fence: people who believe we are too close to the family and people who believe we are too close to the police. If we don't sit in the middle as a completely independent organisation, then we are not doing our job."

The IPCC, which has been operating for seven years, would say that a measure of its increased independence is the rise in the number of investigations it is carrying out alone, rather than acting in a supervisory role over police investigators. Last year, investigators from the police watchdog began 164 independent investigations, 52% more than in 2009-10.

This year, the number of independent investigations is about the same and includes those into deaths after the use of Tasers and pepper spray in the north of England this summer. Jacob Michael, a 25-year-old from Widnes, Cheshire, died after police used pepper spray, and Dale Burns, 27, from Cumbria, was hit three times with a Taser and pepper spray.

Observers note that the organisation is much more transparent than the old, mistrusted Police Complaints Authority and has simplified its procedures, putting more investigators on the frontline. Since 2008, the IPCC has increased its investigators from 107 to 121.

As well as investigating sensitive and controversial deaths during police operations, the IPCC is involved in high-profile inquiries into allegations of corruption, including one involving the two most senior officers in Cleveland police, the chief constable, Sean Price, and his deputy, Derek Bonnard. Insiders say these inquiries prove that the IPCC does not shy away from difficult cases.

But criticisms and concerns remain about the speed with which the IPCC carries out its work, the quality of some of its investigations, and the continued use of former police officers as investigators.

The head of the IPCC investigations team, Moir Stewart, is a former Scotland Yard commander who was criticised by the IPCC in its inquiry into the shooting of Jean Charles de Menezes.

However, the reliance on former police officers has lessened over the years. Of the IPCC's 121 current investigators, 36 are former serving police officers, eight of them former Metropolitan police officers.

• This article was amended on 13 September 2011. The original said the IPCC was 11 years old. A framework was set out in 2000 but it has been operating for seven years. This has been corrected.